GMC Hybrid 2006 Owner's Manual page 38

CAUTION:
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You can be burned if you spill coolant on hot
engine parts. Coolant contains ethylene glycol
and it will burn if the engine parts are hot
enough. Do not spill coolant on a hot engine.
If the SGCM coolant is more than one inch (2.54 cm)
below the FULL COLD mark, your vehicle must be
serviced by your dealer and a special fill procedure must
be followed.
Notice: Attempting to fill the hybrid cooling surge
tank yourself when the fluid level is more than 1 inch
(2.54 cm) below the FULL COLD mark can damage
your vehicle. Your vehicle must be serviced.
5-10
1. Park the vehicle on a level surface and turn the
vehicle off. You can remove the SGCM coolant
surge tank pressure cap when the SGCM cooling
system, including the SGCM coolant surge tank
pressure cap and SGCM cooling hoses, are
no longer hot. Turn the SGCM coolant surge
tank pressure cap slowly counterclockwise (left)
about one full turn. Wait 30 seconds.